Symmetric refrigerant regulator for flooded multichannel evaporator

ABSTRACT

A regulator of refrigerant for a refrigeration circuit with flooded evaporator. The refrigerant can be distributed to many separate evaporator channels. The flow of refrigerant can be changed so that the evaporator and condenser change functions. This provides the opportunity for a fast defrosting of the evaporator or the evaporator can alternately be applied for cooling and heating. The regulator functions independently of the gravitational field and therefore it can be applied for air-conditioning systems in aeroplane and space crafts. The regulator is without movable parts. It has two throttling steps, e.g. two capillary tubes separated by a suction-gas heat exchanger. It requires neither adjustment nor maintenance and therefore it can be placed at inaccessible places or it can be embedded completely in insulation foam.

TECHNICAL FIELD

The invention relates to a refrigerant circuit with a compressor (A), acondenser (D), a evaporator (C) and a suction-gas heat exchanger whereinthe refrigerant is capillary-tube throttled in two steps, firstly fromthe condenser to the suction-gas heat exchanger (F) and then from thesuction-gas heat exchanger to the evaporator (E).

The purpose of such a circuit is to distribute the refrigerant so thatthe evaporator is flooded, and that the suction gas at the compressorinlet is overheated.

BACKGROUND ART

Such a circuit is known from DK174179 wherein the suction-gas heatexchanger condenses the vapour which reaches the fluid container so thatonly pure liquid is conducted further. Simultaneously the heat exchangerregulates the pressure in the container—and thereby regulates themagnitude of the flow of refrigerant to the evaporator. The magnitude ofthis flow of refrigerant controls the extent of flooding (oroverheating) of the suction gas which controls how strong thesuction-gas heat exchanger cools the condensate between the twothrottling steps. The process is self-adjusting, and when equilibrium isreached, the evaporator is flooded.

BRIEF DESCRIPTION OF FIGURES

FIG. 1: Compressor (A), 4-way valve (B) wherein the direction of theflow of refrigerant can be changed. Evaporator/condenser (C,D) issymmetrical and connected through two nozzles, e.g. two identicalcapillary tubes (E,F) which meets in the suction-gas heat exchanger (H),and heat exchanger with the suction line (G).

FIG. 2 shows a regulator for multichannel evaporator/condenser. In thefigure there is shown 3 capillary tubes (E) for connection to theevaporator and 2 capillary tubes (F) for connection to the condenser.The suction line (G) is guides through the external jacket (H) whichexhibits channels for the capillary tubes.

FIG. 3 shows the temperatures in the suction-gas heat exchanger. Te isthe temperature of the suction gas at the inlet of the heat exchanger,and Tx is the “almost” constant temperature at the liquid side. Tc showshow the temperature of the condenser lies with respect to the heatexchanger.

FIGS. 4 and 5 show calculations of the circuit in anenthalpy-log(pressure)-diagram. The refrigerant is R290, the evaporationtemperature is −25° C. and the condensing temperature is 45° C. Thecircuit in FIG. 4 has a larger load of refrigerant than the circuit inFIG. 5, which pulls the evaporator (EF) further to the left. The linesegment CD is enthalpy transferred by the heat exchanger, and the linesegment (FG) is the corresponding shift of the evaporator towards lowerenthalpy. In FIG. 4 the evaporator contains almost 3 times so muchrefrigerant as the evaporator in FIG. 5.

DISCLOSURE OF THE INVENTION

The invention differs from DK 174179 in that the liquid container islacking. Instead the amount of circulated refrigerant is adjusted to theload conditions by binding excess refrigerant in the evaporator. Thishappens by the excess of refrigerant, through the suction-gas heatexchanger, lowers the enthalpy at the inlet of the evaporator wherebythe ratio between vapour and liquid is shifted—so that the density ofthe refrigerant is increased.

The construction is symmetric and the flow of refrigerant can be changedso that the evaporator and condenser exchange their functions. Thisprovides the opportunity for defrosting of the evaporator—or that theevaporator can be applied for both cooling and heating. The method isindependent of the gravitational field and it can function in aeroplaneswherein the system is turned up side down, and in space craftscompletely without gravitational field.

The method is self-adjusting and without movable parts, and therefore itcan be placed in inaccessible places or it can be embedded completely ininsulation foam.

The invention can be applied with all sizes of systems and with mostrefrigerants—albeit not zeotropic mixtures with large temperature slip,because in this case, the regulation of the enthalpy of the evaporatorwould imply large fluctuations of the temperature of the evaporator.

The New Technical Means (Claim 1):

The throttling means consists of two throttling steps separated by asuction-gas heat exchanger wherein the flow velocity through thesuction-gas heat exchanger is so high that liquid and gas are notseparated.

The two throttling steps can be established by two nozzles, e.g. twocapillary tubes, and the suction-gas heat exchanger by two concentrictubes which fulfil the following two requirements:

The heat transfer property must be sufficient for removing all fluidrefrigerants from the suction gas, under all operational conditions.

The flow velocity, at the condenser side, must be so high that liquidand gas are not separated. This is fulfilled at turbulent flow definedby Reynolds's number being larger than 3000.

The Technical Effect (Claim 1):

The condensate passes the suction-gas heat exchanger as a mixture ofliquid and vapour wherein there is thermodynamic equilibrium betweenpressure and temperature. When the suction gas removes enthalpy from thecondensate, some of the vapour condensates—but no significant change ofthe pressure appears and therefore no change in the temperature either.The suction gas passes the condensate in counter current and is heatedto a temperature close to the temperature of the condensate.

This process is self-adjusting

Proof:

When the refrigeration circuit has an excess of refrigerant, thisresults in a drop in enthalpy at the outlet of the evaporator. Thechange of enthalpy cannot pass the heat exchanger since the suction-gastemperature after the exchanger is “almost” constant—and the drop inenthalpy will therefore be transferred to the condenser side, where theenthalpy at the inlet of the evaporator drops correspondingly.

A drop in enthalpy at the evaporator inlet implies a larger density inthe evaporator—and thereby a binding of the refrigerant—which reducesthe cause—which was excess of circulating refrigerant.

Similarly, when the circuit lacks refrigerant:

This causes an increase of the enthalpy at the outlet of the evaporator.The change of enthalpy cannot pass by the heat exchanger because thesuction-gas temperature after the heat exchanger is “almost”constant—and the increase in enthalpy will therefore be transferred tothe condenser side where the enthalpy at the inlet of the evaporatorincreases correspondingly.

An increase in enthalpy at the inlet of the evaporator implies lessdensity in the evaporator—and thereby the refrigerant is released—whichreduces the cause—which was a deficit of circulating refrigerant.

The construction is symmetrical and the flow of the refrigerant can bechanged so that the evaporator and condenser exchange function.

The New Technical Means (Claim 2):

The invention can easily be extended to regulate systems wherein theevaporator and/or the condenser are partitioned into several sections.

The nozzles to and from the suction-gas heat exchanger can be replacedby more parallel nozzles so that there is a separate nozzle for eachsection in the evaporator/condenser.

The Technical Effect (Claim 2):

The partition into many parallel nozzles does not provide any problemsat the collection of refrigerant from many condenser sections, but thedistribution of refrigerant to many evaporator sections could be aproblem in that some nozzles are supplied much liquid while others aresupplied are supplied much vapour. This problem is solved by requirementof a turbulent flow in the heat exchanger which ensures a homogeneousmixture of liquid and vapour, which can then be distributed to manynozzles without problems.

EXAMPLE

Compressor SC21CNX2 is for R290 and has a power of 750 Watt at theevaporation temperature −25 Celsius and condensation temperature 45Celsius, corresponding to a flow of mass of 3 gram/second. Bothcapillary tubes have a diameter of 1 mm and a length 1000 mm,corresponding to a capacity of 27.4 litre of nitrogen per minute.

FIG. 3 shows that the heat exchanger at maximum must transfer 50% of therefrigeration power, here 400 W, at a temperature difference of 30Kelvin, which requires an area of 90 cm2. The diameter of the suctionline is 10 mm, so 90 cm2 corresponds to the surface of about 30 cm ofthe suction line.

The heat exchanger consists of two concentric copper tubes with length300 mm. The inner tube is the suction line having an outer diameter of10 mm, and the outer tube is chosen with an inner diameter of 10.4 mm sothat the distance between the tubes becomes 0.2 mm. The opening betweenthe tubes becomes 6 mm2, and from this it can be calculated thatReynolds' number lies in the range 3200 and 6000, which ensuresturbulent flow.

SUMMARY OF ADVANTAGES OF THE INVENTION

It is as simple and robust regulator of refrigerant for systems withflooded evaporator, including multichannel evaporators with severalparallel sections.

The direction of flow of the refrigerant can be changed so that theevaporator and condenser changes function.

It does not require adjustment or maintenance and it can be placed atinaccessible places.

It functions independently of the gravitational field and it can beapplied in aeroplanes and space crafts.

1. A refrigeration circuit comprising: a compressor, an evaporator, acondenser, a suction-gas heat exchanger, and a throttling means composedof a pressure -reducing nozzle connecting the bottom of said condenserto said suction-gas heat exchanger, and a pressure-reducing nozzleconnecting said suction-gas heat exchanger to said evaporator, whereinthe flow of refrigerant through said suction-gas heat exchanger isturbulent at the side of the condensate.
 2. A refrigeration circuitaccording to claim 1 wherein the evaporator and/or the condenser arepartitioned in several sections, and wherein each section is connectedto the heat exchanger through a separate pressure-reducing nozzle.
 3. Arefrigeration circuit according to claim 1 further comprising a means(B) for changing the direction of the flow of refrigerant through theevaporator and condenser.
 4. A refrigeration circuit according to claim2 further comprising a means (B) for changing the direction of the flowof refrigerant through the evaporator and condenser.